This invention relates to a telecommunication system where remote user terminals are in receipt of access to a larger telecommunication system by means of direct access to a satellite which, in turn, is in communication with a ground station. It particularly relates to a system where, potentially, a user terminal can access more than one satellite and, via a satellite or satellites, can access more than one ground station.
First, it is necessary to describe the general terrestrial telephone system. This is the xe2x80x9clarger telecommunication systemxe2x80x9d mentioned above.
The terrestrial telephone system comprises the sum of all of the wire, optical, point-to-point, microwave and any other means of conducting signals from one point to another via the surface of the earth, together with their associated switches, amplifiers, encoders, decoders, amplifiers and repeaters. On a national level, the network links all access points in a country. Country is linked to country via international trunk lines. Oceans are spanned by submarine cables. The terrestrial network is a transparent carrier where a signal may be introduced from an access point and routed to another, selected access point elsewhere on the surface of the earth. A subscriber can access the terrestrial system via various gateways. For example, the terrestrial system can be accessed from the public switched telephone network, a public switched data network, an integrated switched digital network, a cellular telephone system, or a satellite telephone system. Each gateway provides a plurality of different access points, which, depending upon the gateway, can range from a simple telephone to a complex data or image source. The subscriber""s gateway, at the subscriber""s request, commands the terrestrial system to route and carry the message from the subscriber to a selected destination. The message comes in through the subscriber""s gateway, passes through the terrestrial system, and exits through the selected destination gateway to reach the access point required. Once the message path is established, two-way communication can commence. This is the way the terrestrial system works. Each gateway is connected to another single gateway. The terrestrial system is not adapted, readily, to switch, during the course of a message, between different source gateways or between different destination gateways.
Each gateway, in what is generally understood to be a telephone system, provides a plurality of access points whereby a subscriber may make or receive calls. Each access point is unique in its identity or location and definitely is unique to its particular gateway. In other words, one access point means one gateway.
Now, satellite systems provide a gateway to the terrestrial system via earth stations, capable of establishing two way contact with a satellite or satellites. In turn, the satellites can communicate with user terminals (portable handsets). The user terminals are thus put in contact with the earth stations which can act as a gateway to the terrestrial system.
Sadly, each earth station is a separate gateway. Generally, the earth stations are widely spread apart. There is envisaged an earth station density of one, two or just a few earth stations on each continent. Thus, in general, each user terminal will see satellites which are visible only to one earth station. This satisfies the terrestrial network requirement xe2x80x9cone access point means just one gateway.xe2x80x9d This situation is not a problem. However, there are circumstances, for example, in the middle of the Pacific ocean, in the middle of the Sahara desert, high atop the Andes, where a user terminal may be able to see a satellite or satellites which, in turn, are visible to more than one earth station. This now breaks the rule xe2x80x9cone access point means just one gatewayxe2x80x9d. The user terminal is an access point. Each earth station is a gateway. Now there is the situation xe2x80x9cone access point-many gatewaysxe2x80x9d. The terrestrial network is not adapted to respond to multiple gateways. Further, the signals to and from the earth station, satellites and user terminals are forever changing. What was a perfectly good satellite link a moment ago is no longer functional because the satellite has moved, the user terminal has been shielded by a building or other structure, and a host of other reasons. However, one of the other user terminal to satellite to ground station links might still work. How can the switch between ground stations be effected without confusing the operation of the terrestrial network? The present invention seeks to solve the twin problem of multiple gateway access and operational gateway switching at minimal cost and with least disruption to the general functionality of the terrestrial network.
According to a first aspect, the present invention consists in a communication system wherein a user terminal can communicate with a plurality of satellites; wherein each of said plurality of satellites can communicate with a plurality of earth stations; wherein each of said plurality of earth stations is operable to provide a gateway into a terrestrial system; and wherein said user terminal is registered at a selected one of said plurality of earth stations as its gateway station; said system being characterised by said gateway station, in the event of being unable to establish contact with said user terminal, being operative to select and instruct another earth station to establish contact with said user terminal to act as an alternative earth station and being further operative to pass messages to and from said user terminal through said alternative earth station while acting as said gateway into said terrestrial system.
The present invention further provides a communication system wherein said gateway earth station is operative to employ a low capacity data link to instruct said alternative earth station to attempt to establish contact with said user terminal and, in the event of said alternative earth station establishing contact with said user terminal, is operative thereafter to employ a high capacity voice link to pass messages to and to receive messages from said alternative earth station.
The present invention further provides a communication system wherein said gateway earth station is operative to recall the last known geographical location of said user terminal and is further operative to select, as said alternative earth station, that earth station most likely to establish contact with a user terminal at said last known geographical location.
The present invention further provides a communication system wherein said gateway earth station is operative to recall and employ a plurality of timed previous geographical locations for said user terminal to estimate the instant position of said user terminal and is further operative to select, as said alternative earth station, that earth station most likely to establish contact with said user terminal at said estimated instant position.
The present invention still further provides a communication system wherein said gateway earth station, in the event of being incapable of establishing contact with said user terminal, is operative to select in turn, as said alternative earth station, each of those earth stations which are geographically most proximate to said gateway earth station until an alternative earth station is found capable of establishing contact with said user terminal.
The present invention still further provides a communication system wherein said gateway earth station is operative to execute a calculation to determine whether or not said gateway earth station is capable of establishing contact with said user terminal and to select said another earth station as said alternative earth station if and only if said calculation indicates that said gateway earth station is incapable of establishing contact with said user terminal.
According to a second aspect, the present invention consists in a method for establishing communication with a user terminal for use in a communication system wherein a user terminal can communicate with a plurality of satellites; wherein each of said plurality of satellites can communicate with a plurality of earth stations; wherein each of said plurality of earth stations is operable to provide a gateway into a terrestrial system; and wherein said user terminal is registered at a selected one of said plurality of earth stations as its gateway station; said method comprising the steps of said gateway station, in the event of being unable to establish contact with said user terminal, selecting and instructing another earth station to establish contact with said user terminal to act as an alternative earth station and said gateway station passing messages to and from said user terminal through said alternative earth station while acting as said gateway into said terrestrial system.
The second aspect of the present invention further provides a method including the steps of said gateway earth station employing a low capacity data link to instruct said alternative earth station to attempt to establish contact with said user terminal and, in the event of said alternative earth station establishing contact with said user terminal, employing thereafter a high capacity voice link to pass messages to and to receive messages from said alternative earth station.
The second aspect of the present invention still further provides a method including the steps of said gateway earth station recalling the last known geographical location of said user terminal and selecting, as said alternative earth station, that earth station most likely to establish contact with a user terminal at said last known geographical location.
The second aspect of the present invention yet further provides a method including the steps of said gateway earth station recalling and employing a plurality of timed previous geographical locations for said user terminal to estimate the instant position of said user terminal and, thereafter, selecting, as said alternative earth station, that earth station most likely to establish contact with said user terminal at said estimated instant position.
The second aspect of the present invention yet further provides a method including the steps of said gateway earth station, in the event of being incapable of establishing contact with said user terminal, selecting, in turn, as said alternative earth station, each of those earth stations which are geographically most proximate to said gateway earth station until an alternative earth station is found capable of establishing contact with said user terminal.
The second aspect of the present invention further provides a method including the steps of said gateway earth station executing a calculation to determine whether or not said gateway earth station is capable of establishing contact with said user terminal, and selecting said another earth station as said alternative earth station if and only if said calculation indicates that said gateway earth station is incapable of establishing contact with said user terminal.
In a preferred embodiment, a plurality of earth stations are each uniquely connected to and controlled by a respective mobile satellite switching centre, the mobile satellite switching centre acting as a gateway into the terrestrial telephone system. The user terminal is preferably provided in the form of a portable telephone handset, capable of sending radio signals to and receiving radio signals from any one of a plurality of satellites, but equally could be a dedicated ship, aircraft, land vehicle or manually portable device. The satellites are preferably in a constellation comprising ten satellites in each of two orthogonal orbits, each orbit being at 45 degrees to the equator of the earth, and each at a height of 10,500 km above the surface of the earth, but equally could be in any configuration capable of presenting access to one or more satellites at a time to the user terminal and of presenting one or more satellites at a time to an earth station for contact to be made between at least one satellite and one earth station. Each mobile satellite switching centre preferably contains a visitor register whereat the user terminal can register as a visitor so that the terrestrial system, when originating a call to the user terminal, can consult a home register which holds details of at which mobile satellite switching centre the user terminal is registered as a visitor, so to route telephone calls directly to that mobile satellite switching centre whereat the user terminal is registered as a visitor.
Each earth station is capable of passing commands to all the other earth stations and of receiving responses back, preferably by means of a low capacity data link, but equally by any means exhibiting the necessary characteristics. The mobile satellite switching centres are provided with means to send voice messages between each other, preferably by means of a dedicated voice grade link, but alternatively by any other means capable of so sending and receiving voice messages.
In its operation, the overall system keeps the mobile satellite switching centre whereat the user terminal is listed as a visitor as the gateway into and out of the terrestrial system and, if it becomes necessary to employ a different earth station to communicate with the user terminal, signals between earth stations and receives voice messages from their associated mobile satellite switching centres so that, even though a different earth station is conducting the communication, the mobile satellite switching centre whereat the user terminal is registered as a visitor remains the sole gateway into the terrestrial system.
When seeking to contact the user terminal, the mobile satellite switching centre whereat the user terminal is registered as a visitor uses various techniques, including geographical location, estimation of geographical location, and surrounding area coverage to select, if required, an alternative earth station to contact the user terminal.